Last week, astronauts on the International Space Station had an unwelcome visitor. A piece of space debris came out of nowhere to buzz the station at high speed. The debris was spotted too late for the space station to dodge it. So the six astronauts on the station were told to "shelter in place." They took cover in two Russian spacecraft docked to the station in case they had to make a hasty departure.

The debris, of unknown origin, passed by without incident last Tuesday morning. But it had a 0.3 percent chance of impact and came within about 250 meters of the station—way too close for comfort.

Orbital debris is a big problem, and there's currently no real solution. There are currently hundreds of thousands of pieces of debris in space—spent rockets, dead satellites, stray pieces of hardware. But only a small fraction are being tracked. A Chinese missile test in 2007 blew up a satellite and created thousands of pieces of debris. And an accidental collision in 2009 between two other satellites further littered the landscape. So we've proven that we're good at making a mess in space. Now someone just needs to figure out how to clean it up. Soon.

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